Former NFL star Aaron Hernandez seemed excited about his acquittal and did not appear to have thoughts of suicide before he killed himself in prison, a report from prison officials says.

The state Department of Correction report also said there were no drugs in Hernandez's system when the former New England Patriots tight end killed himself.

He died at the Souza-Baranowski prison in Massachusetts on April 19 while serving a life sentence for a 2013 murder.

Hernandez's body was found hanging in his cell just five days after he was acquitted over the killings of two men in Boston in July 2012.

The report described interviews with inmates who said that, in the days before his death, Hernandez appeared happy about his acquittal and seemed to be looking forward to the future.

"They stated that he was positive and even happily emotional, which was not usual of Hernandez," the report states.

Other inmates said he had become increasingly spiritual during his time in prison.

"They felt that his suicide had been some sort of religious message," the report said.

The prisons department report also says Hernandez knew about Massachusetts case law that says a prisoner's convictions can be erased if he dies before his appeal has been heard, as Hernandez did.

One inmate said he had recently mentioned that legal principle to him.

"The rumour was that if an inmate has an open appeal on his case and dies in prison, he is acquitted of his charge and will be deemed not guilty," the report said.

An inmate who claimed to be one of Hernandez's closest friends said he was shocked by the suicide because he seemed so upbeat after his acquittal.

"Since Friday's verdict he had been talking about the NFL and going back to play even if it wasn't with the Pats," the inmate said, according to the report.

"He talked about his daughter and spending time with her," the same inmate said.

"There was absolutely no indication he would do anything like that."

A separate report released on Thursday by state police said Hernandez wrote "John 3:16," a reference to a biblical passage, in ink on his forehead and in blood on the wall of his cell.

The Bible verse says: "For God so loved the world, that he gave his only begotten son, that whosoever believeth in him should not perish, but have everlasting life."

Hernandez's right middle finger had a fresh cut and there was blood on adjacent fingers.

There also appeared to be a large circular blood mark on each of his feet. A Bible was nearby, open to John 3:16, with the verse marked in blood.

Hernandez's lawyers in his double-murder trial have also said he showed no signs he planned to kill himself.

They have pledged to conduct an independent investigation into Hernandez's death.

In a statement, the defence team blasted state officials for leaks to the media of some of the information contained in the reports.

"The unprofessional behaviour of those entrusted to impartially and professionally conduct an investigation into Aaron's death has caused grave concern as to the validity and thoroughness of the investigation," the lawyers said.

"Accordingly, we intend to fully, completely and impartially review all of the evidence in this matter."

Hernandez, 27, played three seasons for the Patriots before he was released by the team hours after his arrest in June 2013 over the killing of Odin Lloyd.

The semi-professional football player was dating the sister of Hernandez's fiancee.

Hernandez was convicted of first-degree murder in that case and sentenced to life in prison without the possibility of parole.

In 2014, he was charged over the drive-by shootings of two Boston men in 2012. He was acquitted over those killings last month.